A flat-panel display device such as a liquid crystal display (LCD) device, an organic light emitting diode (OLED), etc. includes a number of optical films or optical resin sheets made of a synthetic resin, and thus its applications have also increased.
Unpolarized light emitted from a light source of a flat-panel display device passes through a polarizing plate so that only linearly polarized light is incident on liquid crystal cells. In this case, the intensity of light may be adjusted and gray scales ranging from black to white may be expressed, depending on a degree of rotation of the polarization axis of incident light. That is, the polarizing plate is one of core parts that can visually determine images implemented on the flat-panel display device.
Generally, an LCD includes two polarizing plates, and further includes a retardation film for light compensation, and an OLED includes one polarizing plate having a retardation film installed therein to prevent reflection of light from an external light source.
The polarizing plate is bonded to an LCD and OLED panel by means of a sticking agent. In this case, it is basically desirable that the sticking agent has a smaller change in retardation value upon reliability evaluation of heat shock (H/S) while exhibiting durability characteristics such as heat resistance, wet-heat resistance, etc.
To improve heat resistance and wet-heat resistance associated with the retardation value of the retardation film, there are various attempts to change materials or form a variety of stack structures, etc.
As an attempt to changes materials, Korean Unexamined Patent Application Publication No. 2010-0026928 discloses a retardation film having excellent heat resistance and durability. Here, the retardation film includes a blend resin which includes an acrylic copolymer resin including an alkyl methacrylate-based monomer and an acrylic monomer other than the alkyl methacrylate-based monomer, a resin having an aromatic ring or an aliphatic ring in the polymer main chain thereof, and a copolymer resin including an aromatic vinyl-based monomer and an acid anhydride-based monomer.
International Publication No. WO 2013/173961 discloses a retardation film including a cellulose acylate satisfying an average degree of substitution (DS) of acyl groups ranging from 2.0<DS<2.6, and a certain polycondensation ester or a certain sugar ester. Here, a polarizing plate has excellent durability under wet-heat environments since an interlayer includes a polyvinyl alcohol resin, or an acryl resin containing a polar group.
As a technique of a structural change, Korean Unexamined Patent Application Publication No. 2014-0144695 proposes a retardation film having a multi-layered structure of a retardation layer, an interlayer and a support. Here, the retardation film has excellent durability due to the use of an additional film as the support.
Although the durability or wet-heat resistance characteristics of the retardation film may be improved to some extent using the techniques disclosed in the patents '928 and '961, the retardation film prepared by elongation may be shrunk. Also, in the case of the patent '695, it is impossible to obtain a thin polarizing plate of a polarizer due to inclusion of an additional layer as an interlayer.